Natural gases and liquid (liquified) hydrocarbons are known to contain acid gases such as carbon dioxide and one or more sulfur containing components such as carbonyl sulfide, hydrogen sulfide, mercaptans and the like, many of which must be removed to make the hydrocarbons suitable for the many uses, such as polymerizations, combustion, and the like, because of the deleterious effect these gases have on catalysts, air quality, etc.
Carbonyl sulfide (COS) is contained in natural gas in small quantities i.e., 50-500 ppm and in liquid (liquified) hydrocarbon streams in concentrations in the 1-100 ppm range. The specification level in treated products is in the 1 ppm range or less for natural gas and most liquid streams. There are several processes available for removing COS to these levels. However, some specifications for liquid (liquified) hydrocarbon products are in the 50-1000 ppb range. For example, ethylene and propylene for polyethylene and polypropylene manufacture respectively, normally have a 50 or less ppb limit. There are no known techniques for removing COS to these low levels, e.g. ppb, which are commercially viable. The removal of carbonyl sulfide (COS) is very poor in caustic solutions. No more than 10-15% removal can be expected in conventional caustic solutions used in the conventional designed contactor.
There are, of course, several processes which employ physical solvents and or solvents which aid in the hydrolysis of COS to its component CO.sub.2 and H.sub.2 S, but most of these processes have high make-up rates because the solvents and/or co-solvents are soluble in the liquid hydrocarbons to varying degrees requiring excessive make-up and thus are uneconomical in commercial processes.
In addition most of the materials for removing the COS to the ppm level usually form products which are either impure and command a very poor price or are objectionable from the environmental standpoint.
While mercaptans (RSH) are contained in some natural gases essentially all refinery liquid (liquified) hydrocarbon streams contain mercaptans. The specifications for refinery use and for polymerization reactions employing these products usually require the mercaptan level to be in the 1-20 ppm range.
The known art for the removal of mercaptans consists of one to three stages of contact with an aqueous caustic solution containing some type of solubilizer. Many patents have issued on this concept, most of which have expired. A separate unit is required for these operations. An example of one such process is the Merox Process requiring a separate unit in which the mercaptans are converted to their disulfides. The Merox solution is a caustic solution which contains a proprietary catalyst which is necessary for the economic conversion of RSH to RSSR. The art as practiced does not use unformulated or catalyst containing solution caustic for RSH removal since removal is poor in caustic alone, particularly with respect to removal of the higher molecular weight mercaptans, e.g., butyl and higher alkyl mercaptans.
The advantages and disadvantages of using monoethanolamine MEA in a refinery for removal of acid gases include some of the following: